You can use this tool without having to be online.
In your browser, select file save-as, and save this page as a file.
Double-click that file to open it in a browser on any offline computer.
Alternatively, download the file from the latest GitHub release - https://github.com/iancoleman/eip2333-tool/releases/latest/
Get the source code from the repository - https://github.com/iancoleman/eip2333-tool
This tool uses several third-party libraries, all open source.
The list of libraries used by this tool can be found in libs/package.json
The list of all dependencies of those libraries can be found in libs/package-lock.json